1. Field of the Invention
The present invention relates to a method of producing an inflation film, which comprises processing a specific ethylene polymer or a resin composition containing said ethylene polymer at low temperature, an inflation film, and a multi-layer inflation film.
2. Description of the Related Art
Ethylene polymers have widely been used as a packaging material. To these ethylene polymers, for example, mechanical properties such as rigidity, impact strength and the like are required. As a method of improving the strength of a packaging film obtained by an inflation method, for example, a technique of processing at the temperature lower than that of a normal inflation processing by using a specific die gap is disclosed (Japanese Patent Kokoku Publication Nos. 6-9863 and 6-9864).
However, this technique is not preferred because processing at low melting temperature accompanies an increase in viscosity of the resin and an increase in loading of a processing machine and electricity consumption arises. To obtain good processability by lowering the melt density of the resin, there is disclosed that the molecular weight of the resin is lowered to improve the fluidity. In case of a High-pressure low-density polyethylene and a straight-chain low-density polyethylene, disclosed in the publications, the strength of the resulting film is drastically lowered by a decrease in molecular weight. Therefore, it is hardly to say that the mechanical strength reaches a satisfactory level.
It has recently been known that an ethylene polymer wherein the weight proportion of the cold xylene-soluble portion is small, for example, an ethylene polymer, which is produced by using a catalyst containing a transition metal compound having a group with a cyclopentadiene type anion skeleton, i.e. metallocene polymerization catalyst, exhibits excellent mechanical properties. However, such an ethylene polymer produced by using the metallocene polymerization catalyst has a problem such as fish eye at the time of film molding due to poor thermal stability because a torque at the time of processing is high to easily cause shear and heat generation.
On the other hand, high quality such as low odor, low taste or the like has been required to a food packaging material in which the ethylene polymer is widely used.
However, it is hardly to say that a film obtained by molding the conventional ethylene polymer according to a conventional processing method has odor and taste in a satisfactory level. It is generally considered that taste transferred to foods and odor of the packaging film produced by using the ethylene polymer are generated by oxidation of the molten ethylene polymer in an air and thermally decomposition of said polymer when the film is molded by the inflation molding method. One reason of odor and deterioration of taste includes additives to be added to the ethylene polymer, such as antioxidants, lubricants and the like. These additives were added for the purpose of preventing fish eye and blocking of the film, which are caused in the film at the time of processing. In a conventional technique, the addition of these additives was essential.